1. Field of the Invention
The invention concerns a method to determine the actual flip angle of at least one RF pulse of a pulse sequence during continuous travel of the examination subject in a magnetic resonance apparatus, a method to adjust the transmitter voltage for RF pulses during continuous travel of the examination subject in a magnetic resonance apparatus, and a magnetic resonance apparatus to implement such methods, as well as a non-transitory electronically readable data storage medium that causes a processor, in which the medium is loaded, to implement such methods.
2. Description of the Prior Art
Magnetic resonance (MR) tomography is an imaging method that is used in materials research, pharmaceutical development, and primarily in medical diagnostics. In magnetic resonance tomography, the examination subject is exposed to a homogeneous, static basic magnetic field B0. The nuclear spins (abbreviated as spins) of the examination subject align parallel to the field. To generate measurement signals (in particular images), this steady state is initially disrupted by the radiation of radio-frequency pulses (abbreviated as: RF pulse). The field emitted during the return of the pins to the steady state is spatially coded by the switching of gradient fields, and the signals corresponding to this field received with one or more reception coils. An RF pulse generates a field that is designated as the B1 field, which is amplitude-modulated and oscillates with a carrier frequency. The B1 field is oriented perpendicularly to the B0 field. An RF pulses is characterized by its bandwidth δf, its time duration T, and the time curve of its envelope B1(t). The magnetization of spins whose resonance frequency lies within the bandwidth of the RF pulse is flipped out of the steady state at the end of the RF pulse by the angle
                    α        =                  γ          ⁢                                    ∫                              t                0                                                              t                  0                                +                T                                      ⁢                                                            B                  1                                ⁡                                  (                  t                  )                                            ⁢                                                          ⁢                              ⅆ                t                                                                        (        1        )            wherein t0 is the activation time of the RF pulse, and the gyromagnetic ratio γ is a physical constant that depends on the excited nucleus. For protons, its value is γ=2π 42.57 MHz/T.
The angle α (also called a tilt angle or flip angle) significantly affects contrast and signal strength of the images calculated from the received signal. If the flip angle required by the imaging sequence is not achieved, or if the rotation exceeds this flip angle, this leads to contrast and signal losses, the severity of which depends on the sequence technique that is used.
The B1 field generated by an RF pulse depends not only on the controllable output voltage of the radio-frequency amplifier (in turn, the current through the transmission coil) but also on a load that is dependent on the examination subject, for example a patient-specific load given examinations of patients. Therefore, for a precise determination of the flip angle it is necessary to determine the output voltage of the radio-frequency amplifier (which generates a defined B1 field for a normalized reference RF pulse, and therefore a desired flip angle α of the magnetization) for each examination subject and for every position of the examination subject in the basic magnetic field, for example in what is known as a “transmitter adjustment”. The result of a “transmitter adjustment” is also designated in the following as a reference transmitter voltage, or abbreviated as a reference voltage. If the examination subject is a patient, such a determination should take place for each patient, and ideally for each position of the patient bed (and therefore of the patient) that will be adopted for the diagnostic measurement (data acquisition) of the examination. In the following, a patient is discussed as an examination subject. The information analogously applies to other examination subjects.
If the duration of an RF pulse or their envelope B1(t) of an RF pulse differs from the duration or the envelope of the reference RF pulse, the output voltage of the radio-frequency amplifier for this RF pulse is scaled relative to the reference voltage according to Equation (1) above.
The time expended for the transmitter adjustment (and possibly other adjustment measurements that must be implemented specific to the patient to ensure a desired image quality) is additive to the total examination duration, and therefore adds to the cost of the MR examination and the stress to which the patient is exposed by the examination.
If measurement now takes place in an examination with varying positions of the patient bed, the adjustment measurements should optimally be implemented repeatedly for each individual position. For this purpose, the respective individual positions would need to be occupied in succession and the patient bed would need to be halted at each position in order for the adjustment measurement to take place, which is extremely time-consuming (and therefore unattractive). For example, this is the case in examinations known as multi-step whole-body or partial-body examinations, and in particular in examinations in which measurement takes place during continuous feed of the patient bed (known as “move during scan” (MDS), “continuously moving table MRI” (CMT) and “syngo TimCT” (Siemens proprietary terminology)).
As used herein “adjustment measurements” encompasses all measurements that are implemented specific to the patient and possibly specific to the bed position in order to be able to produce a fine tuning of the MR system to the specific load. In addition to the transmitter adjustment that is discussed above, they normally additionally may include a tuning of the coils (“coil tuning”) in order to compensate for the influence of the patient on the inductance, the capacitance and the resistance of the oscillating circuit used for reception, a frequency adjustment in order to adapt the RF carrier frequency or center frequency to the resonance frequency of the nucleus under consideration (most often free water); and a “shim adjustment” in order to reestablish the homogeneity of the magnetic field (which homogeneity is disrupted by the person to be examined or by the examination subject to be examined).
In numerous publications about MR measurements that are implemented during continuous travel of the patient bed, patient-specific adjustment measurements are omitted entirely. Instead of this, patient-independent system values or empirically determined experimental values for the load-dependent adjustment values are used, for example, and image quality limitations are accepted as a result. One exception is the work by A. Shankaranarayanan and J. Brittain, “Continuous Adjustment of Calibration Values for Improved Image Quality in Continuously Moving Table Imaging”, Proc. Intl. Soc. Mag. Reson. Med. 11 (2004), #103. The authors describe a modification of the adjustment values during the continuous travel. The adjustment values that are thereby used are determined before the actual measurement, at 16 stations distributed over the complete body in what is known as a “prescan” given a stationary bed.
Known adjustment methods with stationary patient beds are frequently implemented iteratively, meaning that a start voltage is initially selected and the flip angle that is achieved with this is determined with the use of the method. If the flip angle deviates significantly from the desired flip angle of the reference RF pulse (for example 180° or 90°), a new transmitter voltage is extrapolated using the flip angle measured in the preceding iteration step and the desired flip angle, and the method is repeated with the transmitter voltage that is determined in such a manner. The iteration ends when the deviation between measured flip angle and desired flip angle falls below a determined threshold.
A transmitter adjustment during continuous feed of the patient bed is already known from U.S. Pat. No. 7,145,338 B2, whereby the problem described above, namely the need to occupy all individual positions for the adjustment measurements successively and to halt the patient bed for the adjustment measurement, can be circumvented.
Such a method can be used in magnetic resonance apparatuses commercially available from Siemens Healthcare for MR examinations in which measurement takes place during continuous feed of the patient bed. The method is not iteratively as under stationary conditions; rather, the transmitter voltage extrapolated with the aid of the start voltage, the desired flip angle and the measured flip angle is set equal to the transmitter reference voltage. The reason is that, due to the continuous travel, the load would change between the individual iteration steps (and thus a convergence cannot be assumed). Further the time duration per adjustment measurement must be constant in order to achieve a predetermined spatial resolution of the transmitter reference voltage as a function of the bed position given a constant speed of the patient bed.
A known method for transmitter adjustment determines the output voltage of the radio-frequency amplifier that is necessary to realize the reference RF pulse by means of a sequence that has three RF pulses, which sequence is shown as an example in FIG. 1.
This method is based on a method that was first proposed by Peter van der Meulen and Gerrit H. van Yperen 1986 at the 5th Annual Meeting of SMRM (Peter van der Meulen and Gerrit H. van Yperen in “A novel method for rapid pulse angle optimization”, Proceedings of the 5th Annual Meeting of SMRM 5th Annual Meeting of SMRM; (1986). p. 1129) and is described in U.S. Pat. No. 4,814,708. The method uses a pulse sequence with three RF pulses as shown in FIGS. 1 and 2 and described in the following.
If α is the flip angle of the first RF pulse, α2 is the flip angle of the second RF pulse and α3 is the flip angle of the third RF pulse, and if τ1 is the time interval between the first and second RF pulses and τ2 is the time interval between the second and third RF pulses, up to five echoes E1, S1, E2, E3, E4 are obtained (see FIG. 1), in particular a first spin echo E1 at time τ1 after the first RF pulse and a stimulated echo S1 at time τ1 after the third RF pulse.
The intensity of the echoes as a function of the flip angle α1 to α3, the time intervals between the RF pulses τ1 and τ2 and the relaxation times T1 and T2 of the examined tissue can simply be calculated analytically. The result for the intensity of the first spin echo is, for example:
                              I                      E            ⁢                                                  ⁢            1                          =                              M            z            0                    ⁢                      sin            ⁡                          (                              α                ⁢                                                                  ⁢                1                            )                                ⁢                                    sin              2                        ⁡                          (                                                α                  ⁢                                                                          ⁢                  2                                2                            )                                ⁢                      ⅇ                          -                                                2                  ⁢                                      τ                    1                                                                    T                  2                                                                                        (        2        )            
The intensity IS1 of the stimulated echo amounts to:
                              I                      S            ⁢                                                  ⁢            1                          =                              1            2                    ⁢                      M            z            0                    ⁢                      sin            ⁡                          (                              α                ⁢                                                                  ⁢                1                            )                                ⁢                      sin            ⁡                          (                              α                ⁢                                                                  ⁢                2                            )                                ⁢                      sin            ⁡                          (                              α                ⁢                                                                  ⁢                3                            )                                ⁢                      ⅇ                          -                                                τ                  2                                                  T                  1                                                              ⁢                      ⅇ                          -                                                2                  ⁢                                      τ                    1                                                                    T                  2                                                                                        (        3        )            
Mz0 is thereby the value of the magnetization at the thermal steady state.
Since the relative value of the flip angles α1, α2, α3 is adjustable via the design of the RF pulses in the sequence (for example via the duration of otherwise identical RF pulses), the desired absolute value of the flip angle can be determined via the measurement of the intensities of at least two echoes. For example, Van der Meulen and van Yperen select all three flip angles to be identical (α1=α2=α3=α) and measure the intensity of the first spin echo and the intensity of the stimulated echo under a constant gradient in the z-direction, as is depicted in FIG. 2. With the aid of Equations 2 and 3, the sought absolute value of the flip angle then results from the ratio of the intensities IS1/IE1:
            I              S        ⁢                                  ⁢        1                    I              E        ⁢                                  ⁢        1              =                                          1            2                    ⁢                                    sin              2                        ⁡                          (              α              )                                                            sin            2                    ⁡                      (                          α              2                        )                              ⁢              ⅇ                  -                                    τ              2                                      T              1                                            =                                                      sin              2                        ⁡                          (              α              )                                            1            -                          cos              ⁡                              (                α                )                                                    ⁢                  ⅇ                      -                                          τ                2                                            T                1                                                        =                                                  1              -                                                cos                  2                                ⁡                                  (                  α                  )                                                                    1              -                              cos                ⁡                                  (                  α                  )                                                              ⁢                      ⅇ                          -                                                τ                  2                                                  T                  1                                                                    =                              (                          1              +                              cos                ⁡                                  (                  α                  )                                                      )                    ⁢                                    ⅇ                              -                                                      τ                    2                                                        T                    1                                                                        .                              
Also:
                    α        =                  arc          ⁢                                          ⁢                      cos            ⁡                          (                                                                                          I                                              S                        ⁢                                                                                                  ⁢                        1                                                                                    I                                              E                        ⁢                                                                                                  ⁢                        1                                                                              ⁢                                      ⅇ                                                                  τ                        2                                                                    T                        1                                                                                            -                1                            )                                                          (        4        )            
The constant gradient Gz in the z-direction (direction of the basic magnetic field B0) has multiple functions: during the RE excitation, it serves as a slice selection gradient that limits the excitation volume in the z-direction. The limitation thereby depends on the amplitude of the gradient and the bandwidth δf of the RF pulses that are used. During the signal reception, the gradient Gz serves as a readout gradient that frequency-codes the echo signals along the z-direction. After a one-dimensional, discrete, complex Fourier transformation of the two echo signals E1 and S1, a one-dimensional, complex slice profile is thus obtained along the z-direction PS1(z) and PE1(z). In Equation 4, the magnitude of the intensity of the central pixel of the respective slice profile is typically used for IS1=|PS1(z0)| and IE1=|PE1(z0)|. This means that the average flip angle is determined in the center z0 of the excitation volume. The term “average flip angle” is used because, due to the lacking a spatial resolution of the method along the two other spatial directions, the echo signals along these two directions are inherently complexly integrated (and therefore averaged). An average T1 value of the tissue in the projection volume is accordingly also to be used for the T1 value in Equation 4.
A modified version of the method just described for transmitter adjustment given stationary measurements is used in DE 10 2005 061 567 B3. The flip angle of the second RF pulse is thereby chosen to be twice as large as the flip angles of the first RF pulse and third RF pulse, thus α1=α3=α and α2=2α. The sought flip angle α is determined from the following Formula 5:
                              cos          ⁡                      (            α            )                          =                                                                              P                                      E                    ⁢                                                                                  ⁢                    1                                                  ⁡                                  (                                      z                    0                                    )                                            ·                                                P                                      S                    ⁢                                                                                  ⁢                    1                                                  ⁡                                  (                                      z                    0                                    )                                                                                                                                          P                                          E                      ⁢                                                                                          ⁢                      1                                                        ⁡                                      (                                          z                      0                                        )                                                                              2                                ⁢          e          ⁢                                    τ              2                                      T              1                                                          (        5        )            Due to the complex multiplication in Formula 5, the method is thus phase-sensitive.
An additional method it to adjust the transmitter voltage given stationary measurements is described in Perman et al., “A Method for Correctly Setting the RF flip angle”, MRM9: 16-24 (1989), for example. The sequence of three RF pulses to generate the echoes E1, S1, E2, E3 and E4 under a constant gradient in the z-direction that is depicted in FIG. 1 is likewise used there, but there the transmitter voltage which is required to generate 90° or, respectively, 180° RF pulses is determined not using the first echo E1 and the stimulated echo S1 but rather only using the third echo E3.
An additional method to adjust the transmitter voltage given stationary measurements is described in Carlson et al. “Rapid Radio-frequency Calibration in MRI”. MRM15: 438-445 (1990). There the sequence shown in FIG. 1 to generate the echoes E1, S1, E2, E3 and E4 under a constant gradient in the z-direction is likewise used, but there the transmitter voltage is determined using the echoes S1, E2, E3 and E4 or, respectively, using the echoes S1, E2 and E4 that arise after the third RF pulse.
The described methods operate well for stationary measurements. However they simply do not deliver satisfactory results for the adjustment of the transmitter voltage given measurements with continuous feed of the patient bed.